1. Field of the Invention
The present invention relates to an image reading apparatus for reading an original image, and particularly relates to an image reading apparatus that can read image while a reading optical member is moved relative to a fixedly placed original.
2. Description of Related Art
Conventionally, a structure as shown in FIG. 4 has been widely used as an image reading apparatus for use in a copying machine or an image scanner or the like.
As shown in FIG. 4, a conventional image reading apparatus is provided with a platen glass 102 on which an original 101 to be read is to be placed, a white plate 103 used for shading compensation and a pressure plate 104 for pressing the original 101.
A lens unit 105 is provided for reduction-imaging reflection light from the original 101 that is illuminated with light, onto a CCD linear image sensor 106.
A first mirror unit 110 is composed of an original illumination lamp 111 in the form of a xenon lamp, a first reflection mirror 112 and an inverter 113 for turning the original illumination lamp 111 on. A second mirror unit 120 is composed of a second reflection mirror 121 and a third reflection mirror 122.
With the above-described structure, the original 101 placed on the platen glass 102 is read while the first mirror unit 110 and the second mirror unit 120 are moved in a scanning manner in the sub-scanning direction (that is, in the right direction in FIG. 4) by a stepping motor 107.
A reader 130 is provided with an adjusting mechanism for adjusting various factors of a read image, such as geometrical characteristics, the magnification or the resolution. Optical adjustment is to be performed by operating this image adjusting mechanism.
As such an image adjusting mechanism, there has been used an adjusting mechanism that adjusts the positions of the second reflection mirror 121 and the third reflection mirror 122 in the second mirror unit 120 along the direction indicated by arrow a in FIG. 4. This type of mechanism is disclosed in Japanese Patent Application Laid-Open No. 2001-222075.
In the following, this adjusting mechanism will be described with reference to FIGS. 5 and 6. Each of FIGS. 5 and 6 includes a plan view, front view, a left side view and a right side view.
Referring to FIG. 5, a second mirror frame 201 is a frame for supporting the second reflection mirror 121 and the third reflection mirror 122. The second mirror frame 201 has sliders 204 and 205 and sliders 206 and 207 provided at positions to be in slidable contact with a front optical rail 251 and a rear optical rail 252 respectively. The front optical rail 251 and the rear optical rail 252 constitute a scanning plane of the second mirror unit 120a. 
The sliders 204 and 205 on one side are fixedly supported on a front slider frame 202. The sliders 206 and 207 on the other side are fixedly supported on a rear slider frame 203.
The front slider frame 202 is fixed by a screw 208 so that the position of the front slider frame 202 can be adjusted along the direction indicated by arrow b relative to the second mirror frame 201. A screw 209 is provided in the same manner.
In the above-described structure, the position of the front slider frame 202 relative to the second mirror frame 201 is adjusted based on geometrical characteristics of the image in order to adjust the inclination of the second reflection mirror 121 and the third reflection mirror 122 with respect to the thrust direction.
The rear slider frame 203 is supported and fixed on the second mirror frame 201. The rear slider frame 203 is constructed in such a way that its rotational position about a shaft 210 provided on the second mirror frame can be adjusted.
The position of the rear slider frame 203 relative to the second mirror frame 201 is adjusted in order to enhance the coplanarity of the sliders 204, 205, 206 and 207.
As shown in FIG. 6, a structure in which a second mirror frame 301 is secured to a front side plate 302 and a rear side plate 303 by means of screws or the like may also be adopted.
In this structure, the second reflection mirror 121 and the third reflection mirror 122 are supported on the second mirror frame 301. The second mirror frame 301 has sliders 304 and 305 and sliders 306 and 307 provided at positions to be in slidable contact with a front optical rail 351 and a rear optical rail 352 respectively. The front optical rail 351 and the rear optical rail 352 constitute a scanning plane of the second mirror unit 120b. 
Details of the sliders 304 and 305 will be described with reference to FIGS. 7A and 7B. FIG. 7B is an enlarged partial view showing a part of FIG. 7A.
As shown in FIG. 7A, the slider 304 is provided with a sliding portion 304a for sliding in contact with the front optical rail 351 and a screw portion 304b provided perpendicularly to the optical rail surface.
The slider 305 has the same structure as the slider 304, namely, the slider 305 is provided with a sliding portion 305a for sliding in contact with the rear optical rail 352 and a screw portion 305b provided perpendicularly to the optical surface.
FIG. 7B is an enlarged view showing the screw portion 304b or 305b. As shown in FIG. 7B, screw hole portions 301a for engaging with the aforementioned screw portions 304b and 305b are formed on the second mirror frame 301.
Furthermore, as shown in FIG. 7B, a recessed portion 304c or 305c for engagement with an adjusting tool 361 is provided on each of the sliders 304 and 305. With this structure, the sliders 304 and 305 are rotated in the direction parallel to the surface of the front optical rail 351 and the surface of the rear optical rail 352 respectively by means of the adjusting tool 361 engaging with the recessed portions 304c and 305c, so that the positions of the sliders 304 and 305 in the direction indicated by arrow b in FIG. 7A can be adjusted relative to the second mirror frame 301.
The positions of the sliders 304 and 305 are adjusted based on geometrical characteristics of the image in order to adjust the inclination of the second reflection mirror 121 and the third reflection mirror 122 with respect to the thrust direction. In addition, they are adjusted in order to enhance the coplanarity of the sliders 304 to 307.
The sliders 304 and 305 that have been adjusted in their positions are fixed to the second mirror frame 301 by adhesive 371 so that the position of the sliders 304 and 305 will not be changed by scanning operation of the second mirror unit 120b. 
However, the above-described prior art mechanism for adjusting the inclination of the second mirror 121 and the third mirror 122 and for adjusting the coplanarity of the sliders suffers from the following problems.
As shown in FIG. 5, it is necessary for that prior art that the second mirror frame 201 for supporting the second reflection mirror 121 and the third reflection mirror 122, the front slider frame 202 and the rear slider frame 202 for supporting the sliders 204 to 207 be constituted as separate members This results in an increase in the number of parts, which causes an increase in cost.
Furthermore, it is necessary for the structure to allow driver access for adjusting the inclination and the coplanarity in the direction toward the side surface of the second mirror unit 120. However, an frame (not shown) of the image reading apparatus is generally present at a position near the side of the second mirror unit 120a, and therefore, the operationality is very low.
On the other hand, the prior art shown in FIG. 6 does not suffer from the above-described problem. However, another problem arises in connection with the prior art shown in FIG. 6, that is, looseness of the sliders 304 and 305 on the second mirror frame 301 generated by the scanning operation of the second mirror unit 120b. 
Recently, with an increase in the speed of the mirror unit, a materials having high sliding ability such as a polyolefin is used as the material of the sliders 304 to 307. Therefore, the adhesion force of the adhesive is made worse to a considerable degree, and the sliders 304 and 305 are easy to be loosened.
Such looseness of the sliders 304 and 305 causes inclination of the second reflection mirror 121 and the third reflection mirror 122, which, in turn, causes deterioration of geometric characteristics of read images.
One may consider to adhere the sliders 304 and 305 with an adhesive having a strong adhesion force. However, in the case that such an adhesive is used, adjustment of the height of the sliders 304 and 305 becomes impossible in the market, in spite that such adjustment is necessary for correcting geometrical characteristics of the image. Consequently, it is very difficult to adopt adhesion of the sliders 304 and 305 using an adhesive having a strong adhesion force.
In addition, in the sliders 304 and 305 and the second mirror frame 301 according to the prior art shown in FIG. 6, there is play in the mount portion of the sliders 304 and 305 to the second mirror frame 301 due to small play or backlash generated between the aforementioned screw portions 304b and 305b and the screw hole portions 301a, even when the adhesion force of the adhesive 371 to the sliders 304 and 305 is strong.
Due to the play in the mount portion of the sliders 304 and 305 to the second mirror frame 301, vibration will be generated upon the scanning operation of the second mirror unit 120b. As a result, for example, displacement in the image position upon reading a monochrome image or misregistration of colors in the case of a color scanner can be caused.
As per the above, in image reading apparatus such as scanners, positional displacement of images or misregistration of colors are generated due to looseness of slider members for performing position adjustment or play in the mount portion of the slider members, so that there arise problems concerning reliability and stability of the image reading apparatus. Therefore, it is desired to prevent the looseness in slider members such as the sliders 304 and 305 and play in the mount portion of the slider members.